The present invention relates generally to measurement of sheet surface characteristics, and more particularly to measurement of gloss and high gloss, on paper using a single device.
One of the parameters used in determining the quality of a surface is the surface luster or the gloss of the surface. For example, in paper production, various grades of paper having different surface gloss are produced to suit various applications. During paper production, it is desirable to periodically or continuously measure the gloss of the surface of the paper to ensure that the paper surface has the desired gloss.
Typically, the surface gloss of paper is measured using a gloss gauge during the last step of paper production before the finished paper, which is manufactured in a continuous sheet, is packaged in the form of rolls. The rolls of paper are then shipped to paper products manufacturers who process the paper sheet in accordance with the intended use.
Certain devices for determining the gloss of paper surfaces comprise an optical system which measures the intensity of a beam of light reflected from the paper surface. Typically, the gloss of the paper surface is determined by comparing its reflectance to the reflectance of a known gloss standard, such as, for example, a glass tile having a polished surface with a known gloss.
Specifically, in measuring the reflectance of the paper surface, light is projected onto the surface, and a sensor which is responsive to the intensity of light is positioned to measure the intensity of the light reflected from the paper surface. The gloss gauge measures the reflectance of a tile surface in the same manner by substituting the tile surface for the paper surface. The reflectance of the paper surface is referenced to the reflectance of the tile surface, thereby providing a measurement of the gloss of the paper surface. In practice, the reflectance measurement of the tile surface is periodically performed, off-sheet and between scans, as the gloss gauge scans back and forth across the paper surface. The gloss gauge is calibrated during each such measurement with the known reflectance of the tile surface.
Two gloss level measurements have evolved from this basic gloss gauging technique under DIN 54502. For regular gloss measurements, measurements are taken using a 75xc2x0 angle for the incident light beam from perpendicular to the measured surface, and for high gloss, measurements are taken using a 45xc2x0 angle for the incident light beam from perpendicular to the measured surface. Thus, if both measurements are desired on the same machine, in the past, two separate DIN standard measuring devices were needed. This double requirement not only causes a slower process, but also involves twice the equipment which must be purchased, maintained, and upgraded, etc. This situation is particularly troublesome considering the only difference between the two standards is the angle of the light beam striking the surface to be measured for gloss.
The present invention addresses the above issues by providing a single gloss sensor which can perform both the DIN gloss measurement and DIN high gloss measurement using the same hardware, and with minimal delay between the two measurements.
In a first embodyment, the invention functions by providing redirecting mirrors which alter the path of the light source used for the gloss measurements from a position which measures gloss (75xc2x0) to a position which measures high gloss (45xc2x0).
In a second embodiment, optical fibers direct measurement light beams to the necessary measurement angles.